A high-brightness discharge lamp such as a high-pressure gas discharge lamp may be used as a light source in an image displaying apparatus such as a projector. To operate the high-brightness discharge lamp, an AC current is supplied to the high-brightness discharge lamp. When the AC current is supplied to allow the high-brightness discharge lamp to light up, in order to suppress the movement of an arc start point or the variation in arc length and to improve the stability of the light arc, it is taught that the absolute value of the AC current supplied to the high-brightness discharge lamp is almost constant and the pulse width ratio of a pulse width of a positive pulse and a pulse width of a negative pulse is modulated (for example, see JP-T-2004-525496).
However, when an AC current whose pulse width ratio is modulated is supplied to the high-brightness discharge lamp, there are problems with the electrodes being excessively melted. These problems are not limited to the high-brightness discharge lamp modulating the pulse width ratio of the AC current, but are common to high-brightness discharge lamps in which the ratio of the power in a positive-electrode period in which one electrode operates as a positive electrode and the power in a negative-electrode period in which the one electrode operates as a negative electrode in one period of the AC current supplied to the high-brightness discharge lamp. These problems are not limited to the high-brightness discharge lamp, but are common to various discharge lamps emitting light by arc discharge between electrodes.